diff --git a/pkg/vm/stackitem/item.go b/pkg/vm/stackitem/item.go
index a737dff29..d928c1a62 100644
--- a/pkg/vm/stackitem/item.go
+++ b/pkg/vm/stackitem/item.go
@@ -33,7 +33,8 @@ type Item interface {
 	Dup() Item
 	// TryBool converts Item to a boolean value.
 	TryBool() (bool, error)
-	// TryBytes converts Item to a byte slice.
+	// TryBytes converts Item to a byte slice. If the underlying type is a
+	// byte slice, it's returned as is without copying.
 	TryBytes() ([]byte, error)
 	// TryInteger converts Item to an integer.
 	TryInteger() (*big.Int, error)
@@ -151,8 +152,11 @@ func convertPrimitive(item Item, typ Type) (Item, error) {
 			return nil, err
 		}
 		if typ == BufferT {
-			return NewBuffer(b), nil
+			newb := make([]byte, len(b))
+			copy(newb, b)
+			return NewBuffer(newb), nil
 		}
+		// ByteArray can't really be changed, so it's OK to reuse `b`.
 		return NewByteArray(b), nil
 	case BooleanT:
 		b, err := item.TryBool()
@@ -519,9 +523,7 @@ func (i *ByteArray) TryBool() (bool, error) {
 
 // TryBytes implements Item interface.
 func (i *ByteArray) TryBytes() ([]byte, error) {
-	val := make([]byte, len(i.value))
-	copy(val, i.value)
-	return val, nil
+	return i.value, nil
 }
 
 // TryInteger implements Item interface.
@@ -982,9 +984,7 @@ func (i *Buffer) TryBool() (bool, error) {
 
 // TryBytes implements Item interface.
 func (i *Buffer) TryBytes() ([]byte, error) {
-	val := make([]byte, len(i.value))
-	copy(val, i.value)
-	return val, nil
+	return i.value, nil
 }
 
 // TryInteger implements Item interface.
diff --git a/pkg/vm/vm.go b/pkg/vm/vm.go
index 92965c80e..cfeeeb235 100644
--- a/pkg/vm/vm.go
+++ b/pkg/vm/vm.go
@@ -663,10 +663,13 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 	case opcode.CAT:
 		b := v.estack.Pop().Bytes()
 		a := v.estack.Pop().Bytes()
-		if l := len(a) + len(b); l > stackitem.MaxSize {
+		l := len(a) + len(b)
+		if l > stackitem.MaxSize {
 			panic(fmt.Sprintf("too big item: %d", l))
 		}
-		ab := append(a, b...)
+		ab := make([]byte, l)
+		copy(ab, a)
+		copy(ab[len(a):], b)
 		v.estack.PushVal(stackitem.NewBuffer(ab))
 
 	case opcode.SUBSTR:
@@ -683,7 +686,9 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 		if last > len(s) {
 			panic("invalid offset")
 		}
-		v.estack.PushVal(stackitem.NewBuffer(s[o:last]))
+		res := make([]byte, l)
+		copy(res, s[o:last])
+		v.estack.PushVal(stackitem.NewBuffer(res))
 
 	case opcode.LEFT:
 		l := int(v.estack.Pop().BigInt().Int64())
@@ -694,7 +699,9 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 		if t := len(s); l > t {
 			panic("size is too big")
 		}
-		v.estack.PushVal(stackitem.NewBuffer(s[:l]))
+		res := make([]byte, l)
+		copy(res, s[:l])
+		v.estack.PushVal(stackitem.NewBuffer(res))
 
 	case opcode.RIGHT:
 		l := int(v.estack.Pop().BigInt().Int64())
@@ -702,7 +709,9 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 			panic("negative length")
 		}
 		s := v.estack.Pop().Bytes()
-		v.estack.PushVal(stackitem.NewBuffer(s[len(s)-l:]))
+		res := make([]byte, l)
+		copy(res, s[len(s)-l:])
+		v.estack.PushVal(stackitem.NewBuffer(res))
 
 	case opcode.DEPTH:
 		v.estack.PushVal(v.estack.Len())